UCRL-JRNL-215027 Analyzing Nuclear Fuel Cycles from Isotopic Ratios of Waste Products Applicable to Measurement by Accelerator Mass Spectrometry S. R. Biegalski, S. M. Whitney, B.A. Buchholz September 6, 2005 Nuclear Science and Engineering Disclaimer This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or the University of California, and shall not be used for advertising or product endorsement purposes. Analyzing Nuclear Fuel Cycles from Isotopic Ratios of Waste Products Applicable to Measurement by Accelerator Mass Spectrometry Scott M. Whitney1, Steven Biegalski1*, Bruce Buchholz2 1. The University of Texas at Austin, PRC-Nucl Engr Teach Lab, 1 University Station Stop R9000, Austin, TX, 78741 2. Lawrence Livermore National Laboratory, CAMS, Livermore, CA 94551 *[email protected] Abstract An extensive study was conducted to determine isotopic ratios of nuclides in spent fuel that may be utilized to reveal historical characteristics of a nuclear reactor cycle. This forensic information is important to determine the origin of unknown nuclear waste. The distribution of isotopes in waste products provides information about a nuclear fuel cycle, even when the isotopes of uranium and plutonium are removed through chemical processing. Several different reactor cycles of the PWR, BWR, CANDU, and LMFBR were simulated for this work with the ORIGEN-ARP and ORIGEN 2.2 codes. The spent fuel nuclide concentrations of these reactors were analyzed to find the most informative isotopic ratios indicative of irradiation cycle length and reactor design. Special focus was given to long-lived and stable fission products that would be present many years after their creation. For such nuclides, mass spectrometry analysis methods often have better detection limits than classic gamma-ray spectroscopy. The isotopic ratios 151 149 244 Sm , Sm , and Cm were found to be good indicators of fuel 146 Sm 146 Sm 246 Cm cycle length and are well suited for analysis by accelerator mass spectroscopy. Introduction The production of fission products and transuranics in a reactor system is dependent upon specific features and physical parameters of the reactor system such as reactor fuel type, moderator, and overall design. Some of these features may be determined by the isotopic profiles of waste from a reactor. Analysis of the fission products may reveal history of the fuel such as time spent in the reactor core, and in some cases, even the type of reactor from which the spent fuel originates. Since ratios of important nuclides may vary depending on the length of a reactor cycle, then assessing the length of reactor cycles may be quite important in some situations. Often times key fuel cycle indicators such as plutonium and uranium are not available (e.g., after reprocessing). Considering such a situation, this work establishes which long- lived isotopes are most illustrative of fuel cycle history. It is shown that analysis for specific isotopic ratios contained in the waste after spent fuel reprocessing is indicative of the length of the fuel cycle as well as reactor type when sensitive detecting techniques are utilized. It is important to note that isotopic ratios are utilized in contrast to ratios of different elements since isotopic ratios are least affected by chemical fractionation. Furthermore, conclusive evidence was found expressing the need for sensitive analytical techniques if the spent fuel is to be accurately analyzed for the entirety of information it can provide. Particular isotopes in the spent fuel have characteristics that prevent accurate analysis via conventional techniques such as gamma-ray spectroscopy. Analysis of samples that are small or diluted via environmental transport further 1 exacerbates the hardships of analyzing these particular isotopes and necessitates the use of mass spectrometry detection techniques [1]. Accelerator mass spectroscopy (AMS) is one analytical technique with the capabilities of measuring as little as 105 atoms [2,3]. AMS is also particularly useful for analysis of isotopes with long half-lives, between 10 and 107 years, and low gamma-ray yields. The data presented in this manuscript shows that in many instances radionuclides with low gamma-ray yields are very informative of reactor cycle parameters. AMS requires one to obtain only a very small sample size, on the order of micrograms in some cases, and enables the analysis of isotopes immeasurable by other analytical techniques [2,3]. Most AMS studies with isotopes of interest to the nuclear fuel cycle have examined environmental releases or biological exposures to uranium and plutonium[4], with a few limited forays with 63Ni[5], 90Sr[6], 99Tc[7], and 129I[8]. Our work sorts through the other radionuclides produced in the nuclear fuel cycle to determine which of these radionuclides have the half-life and chemistry suitable for AMS analysis. The isotopes that are most informative of particular parameters of the reactor cycle in which they were produced were determined. A hierarchy of those isotopes that are most useful was produced and the top isotopes were identified. The methods produced for this work identify isotopic ratios that may be used to distinguish commercial power fuel cycles from other fuel cycles. ORIGEN Modeling 2 Simulations of several different reactor cycles were performed with the Oak Ridge Isotope Generation and Depletion codes, ORIGEN-ARP and ORIGEN-2.2 [9, 10]. These codes were used to model the reactor cycles of the Pressurized Water Reactor (PWR), Boiling Water Reactor (BWR), Canada Deuterium Uranium Reactor (CANDU), and Liquid Metal Fast Breeder Reactor (LMFBR). Parameters of each reactor cycle were specified such as power and initial composition. Other parameters of the cycles like irradiation time were varied throughout our study. Simulations were initiated with ORIGEN-ARP to gain a general knowledge of which isotopes to focus on with PWR and BWR cycles. ORIGEN 2.2 was later used to model the PWR, BWR, and also the CANDU and LMFBR. The focus of this study was to determine which isotopes best differentiate between the variables of reactor cycles. Low and high burnup conditions were modeled to identify which isotopic concentrations differed most. A low burnup or “short” irradiation cycle simulation was approximately one month. The high burnup or “long” irradiation cycle was 540 days (18 months), and is typical of a commercial light water power reactor. Obtaining a sample immediately after irradiation is unlikely, so each calculation assumes either a “short” or “long” fuel cycle and a decay period of one year. Input decks for the ORIGEN-2.2 code were written to simulate long and short cycles for each reactor design. The input decks were very similar but differed slightly depending on the reactor. The ORIGEN code that was used to model the reactors varied with respect to power, cross-section libraries, and initial compositions. Depending upon the reactor being 3 simulated, different enrichments of 235U and the absence or presence of initial Plutonium, was specified in the input deck of the code. The ORIGEN codes are a well substantiated set of codes that have been used for decades in nuclear engineering research. According to U.P. Jenquin and R.J. Guenther [9], the ratio of ORIGEN-2 code calculated values of Uranium and Plutonium isotopes to measured values from actual spent fuel to be in the range of 0.94 to 1.03 for the PWRU cross-section libraries*. A similar study by T. Adachi et al. [24] shows other actinide calculations to reveal no significant difference within about 10% of measured values from spent fuel. A combination of noble gas mass spectrometry, reactor modeling and further data analysis corroborated declared fuel burnup to within 4% from a light water reactor [11]. Further studies by Hermann and DeHart have found “reasonable agreement between measured and predicted isotopic concentrations for important actinides,” for BWR spent fuel [12]. A separate study comparing benchmark isotopic samples to the ORIGEN-S codes found that “the code accurately models a broad spectrum of problems to which it is typically applied,” [13]. Since our purposes in this project focus on order of magnitude types of values, any small differences in concentrations will not be significant during any actual analysis. Methods of Analysis The primary goal of this study was to determine the isotopic pairs that are most useful for differentiating between cycle length and reactor design. Knowledge of these * PWRU was the cross-section library used in ORIGEN 2.2 for PWR calculations. 4 parameters may give key insight into the source of fission products found with unknown origin. Using isotopic pairs rather than a single isotope of an element eliminates the discrepancies in many of the initial conditions such as total amount of initial composition and power. Analysis of isotopic pairs also minimizes defects due to variations in chemical separation efficiencies between different elements. The isotopic pairs that are most informative for this study are those pairs that show the most dramatic differences in production with respect to the short or long cycle. To quantify this idea, the output of each reactor simulation was analyzed to form a ratio of isotopic pair ratios.